1. Field of the Invention
This invention relates to a method and an apparatus for producing an anisotropic rare earth magnet, and in particular to a method and an apparatus for producing an anisotropic rare earth magnet of R-Fe-B system (R and Fe are shown on behalf of rare earth metals of lanthanum series and transition metals including iron respectively, B is shown on behalf of other additional metals including boron to improve the properties) represented by a magnet of Nd-Fe-B system.
2. Description of the Prior Art
The magnet of R-Fe-B system is provided in two types as mentioned hereunder;
(a) a sintered magnet which is made into an anisotropic magnet through a process of casting the molton base alloy into an ingot, pulverizing the ingot into fine powder, molding the powder into a green compact by pressing in a magnetic field using a metal mold, and sintering the green compact, and
(b) a super-quenched magnet which is made by using a super-quenched magnet material 57 given with a magnet anisotropy through a process of making a thin flake by cooling super-rapidly the molton base alloy, molding a compacted material 54 with magnetic isotropy by hot pressing at a temperature of about 700.degree. C. (for example, the compacting pressure is 1 ton/cm.sup.2) with a die 52 and an upper punch 53 shown in FIG. 12(b) using the coarse grained powder of said thin flake of the base alloy directly or, as shown in FIG. 12(a), using a green compact 51 having a theoretical density ratio of about 80% molded by cold-pressing the powder of said base alloy (for example, the compacting pressure is 4 ton/cm.sup.2), and performing plastic working of a reduction ratio of area of not less than 40% on the compacted material 54 at a temperature of not higher than 900.degree. C. using a different die 55 and upper punch 56 shown in FIG. 12(c) (for example, the extruding pressure is 4 ton/cm.sup.2).
Applying these magnets having excellent magnetic properties to especially small-sized electric motors used for various automatizing apparatus is very useful to make the motors lighter and smaller, nevertheless the fact is that said magnets are not applied to the motors sufficiently at the present time because of technical problems in the practical application.
In order to apply the aforementioned rare earth magnet to said motors, it is desirable to make the magnet into a thin sleeve or ring-shaped magnet given with magnetic anisotropy in the radial direction. However, in the aforementioned sintered magnet, it is difficult to give a magnet field in the radial direction at the time of forming the powder in a magnetic field, therefore, there is a problem since the anisotropic degree becomes low down to about 50.about.60% of the case of a plate-shaped magnet. And there is another problem in that the sintered magnet is easy to crack owing to anisotropy of the thermal expansion caused by heating and cooling at the time of sintering.
In the super-quenched magnet, it is possible to give the magnetic anisotropy to the utmost limit in even case of said sleeve or ring-shaped magnet because the magnetic anisotropy is given by the plastic deformation without forming in the magnetic field. However, there is a problem in that the heating processes of two cycles are required, that are the molding of the compacted material 54 having magnetic isotropy and theoretical density ratio of not lower than 99% by the die 52 and the upper punch 53 shown in FIG. 12(b), and the forming to give the magnetic anisotropy by the plastic deformation using the die 55 and the upper punch 56 shown in FIG. 12(c). And the magnetic property of this material is affected sensitively by the grain size, therefore there is another problem since the magnetic property deteriorates by the growing of the grain size caused by heating for a long time. In addition to above said magnetic material is quite brittle, therefore forming cracks 58 as shown in FIG. 12(c) are easy to appear in case of forming the compacted material 54 into the sleeve or the ring-shaped magnet material 57 by extruding.